Transmission line techniques for MRI catheter coil miniaturization and tuning

ABSTRACT

Transmission Line Techniques for MRI Catheter Coil Miniaturization and Tuning. The present invention provides a device and method for miniature and tunable MRI receiver coil for catheters that can be used in minimally invasive procedures and intravascular imaging. An MRI receiver coil for catheter procedures is provided having an impedance matching element that includes at least one miniature transmission line cable which are interconnected to construct the impedance matching element. In a particular embodiment, the miniature transmission line cables are constructed to make an inductance matching element defining an inductance L. In another particular embodiment, the miniature transmission line cable is a capacitance matching element defining a capacitance C. The present invention provides a system and method that allows fine-tuning locally with a higher signal-to-noise ratio. Transmission line cables also overcome the minimum size limits of fixed components. The shielded and balance techniques further reduce noise and improve safety.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is cross-referenced to and claims priority fromU.S Provisional application 60/206,458 filed 05/22/2000, which is herebyincorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was supported in part by grant from the NationalInstitutes of Health under grant number 1R01HL61864 The Government hascertain rights in the invention.

FIELD OF THE INVENTION

[0003] This invention relates generally to magnetic resonance imaging(MRI). More particularly, the present invention relates to a device andmethod for MRI receiver coil miniaturization and tuning.

BACKGROUND

[0004] In vivo imaging of arterial plaques poses a significant challengefor resolution and signal-to-noise. Conventional magnetic resonanceimaging (MRI) uses receiver coils placed on the surface of orsurrounding the body to attain resolutions on the order of 1-5 mm.Important anatomical information for arterial plaques, for instance, canbe obtained if the resolution can be extended to 100-500 μm. Since thevoxel volumes will be 100 times smaller, the coil must provide asignificant boost in sensitivity. This can be achieved usingintravascular receiver coils-micro-coils that are inserted by cathetersto the arterial plaque.

[0005] Several design issues are unique to intravascular coil design.First, the coil, matching network and cable must be small enough andflexible enough to pass through larger vessels to the target regionwithout undue trauma to the vessel. The probe cannot completely blockblood flow nor dislodge the plaque. Blood flow will subject the probe tomotion or vibration a problem that is reduced by real-time MRI.Secondly, the relative orientation of the target artery with respect tothe main magnetic field limits the coil configuration that can generatea B1 field local to the plaque. This orientation can be unpredictablefor tortuous vessels such as the coronaries or aortic artery arch, butquite simple for vessels such as the carotids, iliofemoral and poplitealwhich are oriented mainly along the head to foot axes. Finally, theregion of interest lies outside the coil, inspiring the term, inside-outMRI.

[0006] The signal and noise tradeoffs and design principles for MRIreceiver coils are well understood. To detect an MRI signal, a coil mustbe capable of generating an RF magnetic field component B1 orthogonal tothe static field component B0. According to reciprocity, the B1 spatialbehavior determines the sensitivity profile of the coil. The peak B1scales inversely with coil radius but also diminishes outside the coilover a similar size scale.

[0007] The prior art describes several catheter coils such as, forinstance, opposed solenoids, miniaturized versions of saddle and surface(loops) coils, multiple coils, shortened twin lead designs and dipoleantenna designs. In conventional systems, tissue conductivityinductively couples with the receiver coil to generate a resistance,hence noise, that scales approximately with the field of view volumeseen by the coil. In the case of a surface coil, which is a simple tunedcopper loop 3 to 5 inches in diameter, the depth of sensitivity islimited approximately to the coil diameter. Such a coil could image anartery 2.5 inches deep, but couples so much tissue noise that theresolution is inadequate for plaque imaging.

[0008] For very small coils, the resistance becomes vanishingly small,and the wire resistance of the coil becomes the dominant noise source.The resistance varies inversely with the coil quality factor Q whichtends to be fixed by size and geometry. One can increase the number ofturns N to maintain coil size without adverse changes in Q orsensitivity. Furthermore, Q is optimized when the turn spacing is aboutequal to wire radius. Unfortunately, small coils with many turns or inclose proximity to tissue have an associated quasi-static electric fieldthat fringes into the tissue. The fringe field creates an extraresistance due to dielectric loss that can significantly degradesignal-to-noise-ratio. In standard MRI coils, the electric fields areminimized by splitting the coil into segments with extra seriescapacitors but this becomes impractical in small coils.

[0009] Accordingly, there is a need to overcome current problems forconstructing catheter MRI coils that can be used in minimally invasiveprocedures and intravascular imaging.

SUMMARY OF THE INVENTION

[0010] The present invention provides a device and method for miniatureand tunable MRI receiver coil for catheters that can be used inminimally invasive procedures and intravascular imaging. An MRI receivercoil for catheter procedures is provided having an impedance matchingelement. The impedance matching element includes at least one miniaturetransmission line cable which is interconnected to construct theimpedance matching element. In the present invention transmission linecables could also be miniature coaxial cables. In a particularembodiment, the miniature transmission line cables are constructed tomake an inductance matching element defining an inductance L. In anotherparticular embodiment, the miniature transmission line cable is acapacitance matching element defining a capacitance C. Furthermore, thepresent invention includes adjusting the length of at least oneminiature transmission line cable to adjust capacitance C of thecapacitance matching element. In addition, the present inventionincludes adjusting the length of at least one miniature transmissionline cable to adjust inductance L of the inductance matching element.The present invention includes various different geometries ofconnecting the miniature transmission line cables or miniature coaxialcables. For instance, the miniature transmission line cables could beconnected in series or in parallel. In addition, the miniaturetransmission line cables could be connected at one end or at both ends.The miniature transmission line cables could also be construed as anopen circuit or a closed circuit. Furthermore, the various connectionscould be surrounded by a shielded element. The impedance matchingelement comprises conductive thin film layers to form electricallyshielded structures or Faraday shields. These electrically shieldedstructures are, for instance, but not limited to, constructed of silverpaint and coaxial shields. The impedance matching element alsoincorporates balanced transmission lines to prevent common mode current.Furthermore, the present invention includes a fine-tuning element thatincludes at least one miniature transmission line and which is placed inseries with the impedance matching element and connected at both ends.The fine-tuning element could have different electrical properties. Inaddition, the fine-tuning element could be placed remotely from the areaof interest. The present invention also provides the method ofconstructing an MRI receiver coil for catheter procedures that has animpedance matching element. The method steps for constructing such a MRIreceiver coil include the trimming of at least one miniaturetransmission line cable and subsequently connecting the trimmedminiature transmission line cables to construct the impedance matchingelement.

[0011] In view of that which is state above, it is the objective of thepresent invention to provide miniature and tunable MRI receiver coilsfor catheters in minimally invasive procedures and intravascularimaging.

[0012] It is another objective of the present invention to overcomestandard component size limits for constructing catheter MRI coils.

[0013] It is yet another objective of the present invention to augmentor replace lumped capacitors and inductors with transmission line cablesor micro-coaxial cables that can be trimmed to arbitrary length yieldingadjustable component values.

[0014] It is still another objective of the present invention to provideshort circuit or open circuit transmission lines stubs.

[0015] It is still another objective of the present invention to provideflexible MRI receiver coils that have small cross-section diameter sothat they can be used in minimally invasive MRI procedures.

[0016] It is another objective of the present invention to useconductive thin film layers to form electrically shielded structures forintravascular/catheter MRI coils.

[0017] It is another objective of the present invention to shield thepatient and probe from each other and to form a structure that preventsdangerous common mode current and reduces noise.

[0018] It is another objective of the present invention to provideseries connected lines that allow for coil tuning to be approximatelytuned with fine tuning placed remotely in larger cross-sectional areas.

[0019] Most prior art designs use fixed capacitors and must includefine-tuning adjustments about 1.5 meter away from the probe. Capacitorsdo not come in custom sizes for tuning. The advantage of the presentinvention over the prior art is that the system and method enables oneto include transmission line stubs that allow fine-tuning locally with ahigher signal-to-noise ratio. Transmission line stubs also overcome theminimum size limits of fixed components. The shielded and balancetechniques further reduce noise and improve safety.

BRIEF DESCRIPTION OF THE FIGURES

[0020] The objectives and advantages of the present invention will beunderstood by reading the following detailed description in conjunctionwith the drawings, in which:

[0021]FIG. 1 shows an exemplary electrical circuit with a transmissionline according to an embodiment of the present invention;

[0022]FIG. 2 shows exemplary transmission line cables according to anembodiment of the present invention.

[0023]FIG. 3 shows an exemplary embodiment of an electrical circuit withstandard components and transmission lines according to an embodiment ofthe present invention; and

[0024]FIG. 4 shows an exemplary embodiment similar to FIG. 3 with thedifference that a fixed capacitor is replaced by a transmission lineaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Although the following detailed description contains manyspecifics for the purposes of illustration, anyone of ordinary skill inthe art will readily appreciate that many variations and alterations tothe following exemplary details are within the scope of the invention.

[0026] Accordingly, the following preferred embodiment of the inventionis set forth without any loss of generality to, and without imposinglimitations upon, the claimed invention.

[0027] The present invention provides a device and method for miniatureand tunable MRI receiver coil for catheters that can be used inminimally invasive procedures and intravascular imaging. In the presentinvention, lumped capacitors and inductors are replaced or augmentedwith transmission line cables that can be trimmed to arbitrary lengthyielding adjustable component values. These transmission line cablesappear either as short circuit or open circuit transmission line stubs.They have small cross-sectional diameter and are flexible so they can beused in minimally invasive MRI procedures. A transmission line cable is,for instance, but not limited to, a miniature coaxial cable or abalanced shielded line.

[0028] In the design of the MRI receiver coil, the present inventioninvolves an impedance matching element that is build with at least oneminiature transmission line cable. FIG. 1 shows electrical circuit 100that could include at least one miniature transmission line cable. FIG.1A shows electrical circuit 100 with miniature transmission line cable102 which is placed in series in electrical circuit 100. FIG. 1B showselectrical circuit 100 with miniature transmission line cable 104 whichis placed in parallel in electrical circuit 100. Electrical circuit 100could include at least one miniature transmission line with differenttopologies. Each topology could have transmission lines that either havea closed or open circuit as well as transmission lines that either areconnected at just one end or at both ends. In addition, electricalcircuit 100 could also include standard electrical components, forinstance, but not limited to, capacitors, coils, inductors, andresistors. In general, electrical circuit 100 could be any configurationin which an impedance matching element is constructed with electricalspecifications that are in accordance with the requirements andspecifications of a particular minimally invasive procedure and/orintravascular imaging procedure. Conductive thin film layers are used toform electrically shielded structures for intravascular/catheter MRIcoils. These utilize silver painted or coaxial shields to form Faradayshields minimizing surrounding tissue interactions. Thin film sectionscan also be used to create capacitors.

[0029]FIG. 2 shows exemplary embodiments of different miniaturetransmission lines. FIG. 2A shows a miniature coaxial cable 200 with ashield 212A and lead 208. Lead 210 in 200 is connected to shield 212A.In this particular example of FIG. 2A, miniature coaxial cable 200 is acoaxial capacitor. Miniature coaxial cable 200 is defined as acapacitance matching element with a capacitance C. The capacitance C ofthis capacitance matching element is adjustable by adjusting the lengthof the miniature transmission line cable or miniature coaxial cable.Leads 208 and 210 form two terminal ends of transmission line capacitor200. FIG. 2B shows a miniature coaxial cable 202 with a shield 212B andlead 212. Lead 212 in 202 is not connected to shield 212B leaving anopen circuit. Lead 214 is connected to shield 212B. At the opposite endof miniature coaxial cable 202, lead 212 is connected to shield 212Bcreating a closed circuit 216. In this particular example of FIG. 2B,miniature coaxial cable 202 is a coaxial inductor. Miniature coaxialcable 202 is defined as a inductance matching element with a inductanceL. The inductance L of this inductance matching element is adjustable byadjusting the length of the miniature transmission line cable orminiature coaxial cable. Leads 212 and 214 form two terminal ends oftransmission line inductor 202. FIG. 2C shows two miniature coaxialcables 204, which is constructed to create a balanced shieldtransmission line or balanced coaxial pair. In 204, miniature coaxialcable 204A has lead 218 and shield 212C and miniature coaxial cable 204Bhas lead 220 and shield 212D. In addition, 204 shows connections 221 ofshield 212C of miniature coaxial cable 204A to shield 212D of miniaturecoaxial cable 204B. In the present invention, balanced and seriesconnected transmission lines are, for instance, used as a means toshield the patient and probe from each other and prevent dangerouscommon mode current and reduces noise. FIG. 2D shows a transmission line206 that is constructed as a shielded twin line. Transmission line 206,has two leads 222 and 224 both shielded by shield 212E.

[0030]FIG. 3 shows an exemplary embodiment of electrical circuit 100wherein a combination of standard components are used with miniaturetransmission line cables or miniature coaxial cables. In FIG. 3A,transmission line cables 300 and 302 are included with differentelectrical properties, such as length and/or characteristic impedance.Transmission line cables 300 and 302 are fine-tuning elements that areplaced in series with an initially constructed impedance matchingelement or an electrical circuit. These fine-tuning elements areconnected at both ends of the transmission line cable. The MRI receivercoil of the present invention provides hereby a fine-tuning element thatis placed remotely. The series connected lines or fine-tuning elementsallow fine tuning to be placed remotely in larger cross-sectional areas.This fine-tuning transmission line could use micro-coaxial cable ofdifferent characteristic impedance than nominal 50-ohm cable. FIG. 3Ashows transmission line cable 304 which is a coaxial capacitor that isconnected to fine-tuning element 302 and coil 306. Shield 304A ofcoaxial capacitor 304 is connected by lead 303 to shield 302A offine-tuning element 302. Lead 304C is connected to shield 304A ofcoaxial capacitor 304. In addition, lead 302B of fine-tuning element 302is connected to lead 304B of coaxial capacitor 304. Finally, coil 306 isconnected to leads 302B of fine-tuning element 302 and 304C of coaxialcapacitor 304. Transmission line cable 300 could potentially beconnected to a connector or additional electrical circuitry could beadded. FIG. 3B shows the electrical equivalent of FIG. 3A whereincapacitor 308 is similar to capacitor 310, although each could havedifferent capacitance values. In addition, FIG. 3B shows the electricalequivalent of capacitor 304 represented by capacitor 312, again eachcapacitor could have different capacitance values.

[0031]FIG. 4 shows an exemplary embodiment that is almost similar toFIG. 3 with the difference that fixed capacitor 308 in FIG. 3A is nowreplaced by transmission line cable 400 as shown in FIG. 4A.Transmission line cable 400 could be, but is not limited to, a coaxialcapacitor, inductor, balanced shielded transmission line or balancedcoaxial pair, or a shielded twin lead as shown in FIG. 2 and discussedabove. An example of a coaxial inductor is shown in FIG. 4B where 402 isa closed circuit wherein one end of lead 404 is connected to shield 408.However, lead 406 is connected to shield 408 but not connected to lead404. An example of a coaxial capacitor is shown in FIG. 4C wherein lead410 is not connected to lead 414, but lead 414 is connected to shield412.

[0032] The present invention has now been described in accordance withseveral exemplary embodiments, which are intended to be illustrative inall aspects, rather than restrictive. Thus, the present invention iscapable of many variations in detailed implementation, which may bederived from the description contained herein by a person of ordinaryskill in the art. All such variations are considered to be within thescope and spirit of the present invention as defined by the followingclaims and their legal equivalents.

What is claimed is:
 1. An MRI receiver coil for catheter procedureshaving an impedance matching element, said impedance matching element,comprising: (a) at least one miniature transmission line cable; and (b)interconnections between said at least one miniature transmission linecable constructed to make said impedance matching element.
 2. The MRIreceiver coil of claim 1, wherein said at least one miniaturetransmission line cable is at least one miniature coaxial cable.
 3. TheMRI receiver coil of claim 1, wherein said at least one miniaturetransmission line cable is an inductance matching element defining aninductance L.
 4. The MRI receiver coil of claim 3, wherein saidinductance L of said inductance matching element is adjustable byadjusting at least one length of said at least one miniaturetransmission line cable.
 5. The MRI receiver coil of claim 1, whereinsaid at least one miniature transmission line cable is a capacitancematching element defining a capacitance C.
 6. The MRI receiver coil ofclaim 5, wherein said capacitance C of said capacitance matching elementis adjustable by adjusting at least one length of said at least oneminiature transmission line cable.
 7. The MRI receiver coil of claim 1,wherein said at least one miniature transmission line cable has at leastone open circuit.
 8. The MRI receiver coil of claim 1, wherein said atleast one miniature transmission line cable has at least one closedcircuit.
 9. The MRI receiver coil of claim 1, wherein saidinterconnections are surrounded by a shielded element.
 10. The MRIreceiver coil of claim 1, wherein said at least one miniaturetransmission line cable is connected in series.
 11. The MRI receivercoil of claim 1, wherein said at least one miniature transmission linecable is connected in parallel.
 12. The MRI receiver coil of claim 1,wherein said impedance matching element comprises conductive thin filmlayers to form electrically shielded structures.
 13. The MRI receivercoil of claim 12, wherein said electrically shielded structures areselected from the group consisting of silver paint and coaxial shields.14. The MRI receiver coil of claim 12, wherein said electricallyshielded structures are Faraday shields.
 15. The MRI receiver coil ofclaim 1, wherein said impedance matching element incorporates balancedtransmission lines to prevent common mode current and reduce noise. 16.The MRI receiver coil of claim 1, further comprising a fine-tuningelement comprising at least one additional miniature transmission lineplaced in series with said impedance matching element and connected atboth ends.
 17. The MRI receiver coil of claim 16, wherein saidfine-tuning element has different electrical properties.
 18. The MRIreceiver coil of claim 16, wherein said fine-tuning element is placedremotely.
 19. A method of constructing an MRI receiver coil for catheterprocedures having an impedance matching element, comprising the stepsof: (a) trimming at least one miniature transmission line cable; and (b)connecting said at least one miniature transmission line cable toconstruct said impedance matching element.
 20. The method of claim 19,wherein said at least one miniature transmission line cable is at leastone miniature coaxial cable.
 21. The method of claim 19, wherein said atleast one miniature transmission line cable is an inductance matchingelement defining an inductance L.
 22. The method of claim 21, whereinsaid inductance L of said inductance matching element is adjustable byadjusting at least one length of said at least one miniaturetransmission line cable.
 23. The method of claim 19, wherein said atleast one miniature transmission line cable is a capacitance matchingelement defining a capacitance C.
 24. The method of claim 23, whereinsaid capacitance C of said capacitance matching element is adjustable byadjusting at least one length of said at least one miniaturetransmission line cable.
 25. The method of claim 19, further comprisingthe step of surrounding said impedance element by a shielded element.26. The method of claim 19, wherein said step of connecting comprisesthe step of connecting said at least one miniature transmission linecable in series.
 27. The method of claim 19, wherein said step ofconnecting comprises the step of connecting said at least one miniaturetransmission line cable in parallel.
 28. The method of claim 19, whereinsaid impedance matching element further comprises conductive thin filmlayers to form electrically shielded structures.
 29. The method of claim28, wherein said electrically shielded structures are selected from thegroup consisting of silver paint and coaxial shields.
 30. The method ofclaim 28, wherein said electrically shielded structures are Faradayshields.
 31. The method of claim 19, further comprising the step ofincorporating in said impedance matching element balanced transmissionlines to prevent common mode current and reduce noise.
 32. The method ofclaim 19, further comprising the step of including a fine-tuning elementcomprising at least one additional miniature transmission line placed inseries with said impedance matching element being connected at bothends.
 33. The method of claim 32, wherein said fine-tuning element hasdifferent electrical properties.
 34. The method of claim 32, whereinsaid step of fine-tuning comprises the step of fine-tuning remotely.